There are three methods discussed in the prior art for initiating a plasma arc discharge and starting a plasma arc torch. These are: high frequency or high voltage discharge; exploding wire; and contact starting. In recent years, several contact starting methods have come into use. Contact starting is advantageous, because it uses relatively low electrical voltage and avoids the cost of high frequency/high voltage discharge equipment and the associated electromagnetic interference.
One arrangement for contact starting a plasma arc torch is shown in U.S. Pat. No. 4,791,268. In this arrangement, a movable electrode, which acts as a cathode, is urged by a bias spring into contact with a fixed nozzle, which acts as the anode. The movable electrode is formed with a piston part slidingly fit within a cylinder (piston chamber) formed in the torch body. The electrode/cathode is automatically separated from the anode in response to the buildup of gas pressure in the piston chamber within the torch head. The gas pressure causes the piston part and the electrode to move against the force of the bias spring, breaking electrical contact between the electrode and the nozzle. A pilot arc is formed by the separation of the electrode and the nozzle. The same gas flow that is used to drive the piston part also feeds the plasma arc.
Another arrangement for contact starting a plasma arc torch is shown in U.S. Pat. No. 4,896,016. In this arrangement, the electrode is also movable, but it is powered by an over-center spring arrangement, actuated by the operator's forefinger or thumb. Contact between the electrode and nozzle is broken by actuation of the over-center mechanism.
A third arrangement for contact starting a plasma arc torch is shown in U.S. Pat. No. 3,242,305. In this arrangement, the electrode is also movable, but it is actuated by a piston axially linked to the electrode. The piston is powered by a flow of cooling water for the torch head. The chamber in which the piston moves is part of the same torch head that contains the electrode and the region in which the pilot arc is formed.
U.S. Pat. No. 4,791,268 also discusses prior art contact starting systems in which the cathode is the electrode and the nozzle through which the plasma jet passes serves as the electrical conductor connecting the electrode to the workpiece. In these systems, the nozzle is spring mounted and slidable with respect to the electrode and is forced into contact with the electrode (usually against the force of a bias spring) when it is pressed against the workpiece. Thus, the electrode, nozzle and workpiece are all in electrical series connection when the current flow is initiated. When the electrode is manually backed away from the workpiece, the nozzle is allowed to separate from the electrode and return to its normal position. Because such systems require that the nozzle be pushed against the workpiece to force the nozzle and electrode into contact, they are hard to control and not suitable for work on delicate workpieces. U.S. Pat. Nos. 2,898,441 and 4,567,346 show specific designs for such a push-start torch head.
The foregoing arrangements have certain other disadvantages. U.S. Pat. No. 4,896,016 avoids the need for a complex electrode actuation mechanism but is not practical for remote-controlled operation as in U.S. Pat. No. 3,242,305, because there is no mechanism that can be actuated by remote control of a flow of fluid acting on a cylinder. Most plasma arc electrodes last for about one hour of operation before replacement is required. The arrangement shown in U.S. Pat. No. 4,791,268 has an electrode that is expensive to replace, because it has a piston part that is formed as part of the electrode. Because a close-fitting piston part must be machined and the entire electrode-piston element must be replaced, the operating costs of this form of torch are relatively high.
In the piston-actuated prior art devices, the plasma flame chamber and the piston chamber are both within the torch head. (In U.S. Pat. No. 4,791,268 they are a single chamber.) Thus, the cylinder-piston mechanism is subject to the elevated temperatures present in the vicinity of a plasma arc. The cylinder-piston mechanism and the surrounding parts are subject to thermal stress, differential expansion and other thermal-related phenomena that complicate design. Moreover, to construct a torch head of this type that is small enough to be conveniently usable, the cylinder must be made relatively small and, consequently, low-powered. Heat changes the dimensions of the copper parts typically used and scale builds up on some moving parts during operation. Both of these increase friction, which may ultimately impair operation of a low-powered cylinder.
The device of U.S. Pat. No. 3,242,305 has many of the same problems. Although it appears to be constructed so that the electrode and the piston are mechanically separable parts, the electrode and the piston are all part of essentially the same thermal mass. Thus, the piston and its associated cylinder must be provided with fluid flow for cooling, complicating design of the plasma arc head and/or must be made of special materials that can accommodate thermal stress and differential thermal expansion. The latter can be a particularly difficult issue for the close fits that are typically necessary for a piston-cylinder combination. Also, when the fluid used for cylinder actuation is water, there is a danger of leaks.
In sum, a design for contact starting a plasma arc that remedies the above described deficiencies of the prior art would be a decided advance and permit more reliable, less expensive plasma arc torch equipment to be made.